1. Field of the Invention
The present invention relates generally to modular power converters, and more particularly to improved control circuitry for allowing the interconnection and control of a plurality of modular power supplies used to drive a common load. The present invention has particular applicability to current mode controlled power supplies, and is operative to automatically disconnect failed modules from the combination, so as to allow uninterrupted operation of the system in the event that one or more individual modules should fail or be removed/replaced in the system. The present invention allows operation of a modular power system in an energy conservation mode wherein modules are automatically switched in and out of operation as required to insure that the operational modules operate at peak efficiency and thereby conserve energy.
2. Brief Description of the Prior Art
Current mode controlled power supplies, sometimes referred to as power converters, are commonly used to provide power for numerous types of electronic apparatus. Such devices typically have a current sharing terminal at which a signal is developed to provide a means of causing several modules operated in parallel to equally share the load, The usual technique for providing this function in a power supply module is to use a current mode pulse width modulator (PWM), such as a UNITRODE UC3842A as the controller for the module, and to provide the PWM error voltage as the current sharing signal. Current mode control in converters is well known and has been previously described in many publications including the UNITRODE application note U-100A, for example. Basically, the difference between an analog of the converter's output and a reference signal is amplified by an error amplifier. The output of the error amplifier, called the error voltage, is in turn compared to a voltage signal which is an analog of the converter's primary current. The comparator performing this function controls circuitry which terminates the primary current pulse when the level established by the error signal is reached. The signal is usually a low level DC voltage in the 1-6 volt range and is referenced to minus input.
With two or more modules connected in parallel, the signal reference, or ground connection, is already common to each module whether it is input or output referenced. By connecting the current mode PWM reference voltages of the modules to each other, the primary currents are forced to be equal to each other, and hence current sharing is achieved. The current sharing signal is produced by the module for comparison purposes, and has the characteristic that, if controlled or altered external to the PWM, it can adjust the module's current output. If this signal is shorted to signal ground or is held at zero potential externally, the module output will go to zero because the current mode PWM will hold the current to zero. Thus, the current sharing terminal can be used as a point to apply a logic low signal to disable the converter.
The availability of modular power supplies and power converters which can be operated in parallel with current sharing has opened up the possibility of an "n+1" approach to power system design. The number "n" is determined by taking the total system power requirement and dividing by the power to be delivered by each individual module and rounding up to the nearest whole number. This is the minimum number of paralleled modules required to provide the required power. The reliability is, however, increased by adding one more module, bringing the total number of modules to "n+1" so that in the event of failure of a single module the remaining "n" modules can continue to provide the system with the required power. Said systems are said to have "n+1" redundancy.
A problem generally common to such systems is that the signal used to produce current sharing is common to all modules and is a point by which a single failed unit can cause the shutdown of the entire system, regardless of the number of modules in parallel. The common current sharing signal is generally a signal which is proportional to the amount of current, and thus power delivered by each module. Since all paralleled modules share the common signal, a failure of one module which causes the signal to go to zero will cause the power delivered by each and every module to go to zero, thus causing the shutdown of the entire system and defeating the reliability that would otherwise be provided by using "n+1" or more modules.
Systems that are provided with redundancy for reliability are often intended to be operated on a continuous basis, such that shutdown of the system is an event that rarely, or never, occurs. Said systems are generally constructed such that power converters and other system components can be replaced without de-energizing the system. This is generally referred to as "hot-plug-in" or "hot-swapping". In such systems a failed part of the system can be removed and replaced without shutting down the system even momentarily.
Another problem common to the approach of using modular power supplies and power converters operating in parallel with current sharing is the reconnection of the current sharing signal of the module being plugged in during the "hot-plug-in" operation. This operation can cause momentary shutdown of the system because the current sharing signal of the module being plugged in is initially zero.
In a system made up of many individual modules, it is often desirable to provide a means for turning on and off individual modules, either by switching off the input voltage supplied from the input power source to an individual module, or by using logic signals, for the purpose of system testing or to conserve energy. The approach of using modular power supplies and power converters operating in parallel with current sharing presents a problem in performing the turning off of the individual module without causing shutdown of the entire system. Similarly, the turning on or off of an individual module may cause momentary shutdown of the entire system. Many prior art circuits intended to be used for providing current sharing of modules have the disadvantage that not all of the modules are configured identically, but instead one is configured as a "master" and the others are configured as "slaves". In such an arrangement of modules, the master provides a control signal to each slave. This type of system is generally prone to total shutdown in the event of the failure of the master. Such circuits are not suitable for use in "n+ 1" redundancy designs.
Another type of circuit uses a small resistor in series with the output of each module to generate a small voltage which can be compared to that of another module. The difference is usually amplified, and a signal is generated to act upon the module's feedback amplifier or sense circuit in such a way as to force current sharing. This type of circuit has the obvious disadvantage of being wasteful of energy when the output current of the system is high, which is the usual case when many modules are paralleled. For example, if the signal generated by the current sense resistor is 100 mV and the power system output is 3.3 V, then three percent of the output power must be wasted to provide the current sense signal. Sometimes such circuits also have a limit to the number of modules that can be coupled together in a sharing arrangement.
One prior art circuit provides circuitry for each module to which the module current sharing signal is connected. The circuit then amplifies any differences in the current sharing signals and generates a signal to drive the module sense circuit. The module output voltage is driven up if its current sharing signal drops below that of the others. The disadvantages of this circuit are (1) high parts count; (2) input to output isolation must be provided; and (3) a separate amplifier that acts on the sense amplifier to alter the response to the module must be provided.
The input voltage to a module suddenly dropping to zero, due to the main power switch failing short and the individual module's fuse subsequently blowing, is probably the most common fault condition in single-ended converter modules. A very simple decoupling circuit can be made which will decouple the current sharing signal of a module which has experienced this failure mode from that of other modules. However, no provision is made in such circuits for disconnecting the current sharing signal for failures during which the current sharing signal goes low and the input voltage remains good.